Abstract: At approximately 9:40 a.m. on September 19, 2006, Employee #1 and his supervisor were trimming a tall tree using a United Rentals JLG Aerial Platform with a 135-foot boom. Both employees had worn harnesses and their lanyards were tied and secured to the basket of the aerial device. Suddenly, the bracing bar supporting the bottom of the basket buckled (bent). The supervisor in the basket, who was controlling the lift, was not injured. However, Employee #1 was thrown onto the mid-rail of the basket's guardrail, struck his knee, and fractured it. He was taken to San Francisco General Hospital, and then transferred to the Kaiser medical center for further treatment and outpatient surgery. Employee #1 was hospitalized due to his injuries. During investigation and inspection of the machine, it appears the bar had been hit by something. However, it could not be determined what, when and how it happened, and who did it. All employees (Electricians and Arborists) that used the machine were interviewed. They indicated that they were not aware of anyone hitting anything with the lift that they could remember. During the investigation, it was revealed that the electricians received instruction in the proper use of the machine, but when the arborists borrowed the machine to trim the tree's branches, they had not been trained in accordance with the manufacturer's operating instructions.